The present invention relates to a method and apparatus for producing a laser pulse having a relatively low initial energy followed by a number of high energy spikes or variation thereof for use, as an example, in drilling workpieces, especially thin workpieces such as metal layers, with holes having dimensionally accurate entrance and exit shapes with close tolerances. The invention can be applied to the high speed manufacture of products requiring fine tolerances, such as injector spray holes, filter screens, cooling apertures, valve seats, dies and molds or various other applications requiring cutting or welding.
Since the 1970""s, laser techniques have garnered increasing interest in the field of materials processing. Early laser techniques for material processing, such as cutting and drilling applications involved using continuous wave or relatively long pulse length (i.e., from about 0.5 to about 20 milliseconds) lasers such as CO2, ruby and yttrium aluminum garnet (YAG) lasers. These systems suffered the drawback of requiring a relatively high radiant exposure of the workpiece and resulted in significant alterations to surrounding material. Consequently, the lasers of these systems were effective cutting tools mainly in applications that did not require a high degree of precision or control.
In the 1980""s, the erbium doped YAG laser yielded encouraging results by demonstrating the capacity to perform as an efficiently drilling laser while incurring relatively low levels of collateral damage to the surrounding workpiece material, provided that low pulse rates of less than about three pulses per second were applied to the target material. These Er:YAG systems, operating in the microsecond pulse duration regime, have been successfully applied with minimal attendant thermal damage to the surrounding material in several areas of application in material processing and medicine. The combination of high absorption, relatively short pulse duration and low pulse repetition rates enables minimization of collateral workpiece damage for those workpieces having high absorption at the Er:YAG wavelength of 2900 nm. However, in addition to the disadvantage of being effective for precision machining use only with a narrow range of workpieces highly absorbent around 2900 nm, these systems also suffer the drawbacks of having low material removal rates arising from the relatively limited average energy output.
Although the removal rate problem may be addressed by increasing the pulse energy or the pulse repetition rate of the laser, enhancing material removal by increasing laser power is accompanied by increased photothermal and photomechanical effects which cause collateral damage in adjacent material, reducing the effectiveness of the laser as a precision machining tool. When used to drill precision holes, such high-energy laser pulses result in the exit hole being produced not by the laser pulses themselves but by molten and/or vaporized workpiece material exploding through the exit surface. The geyser of workpiece material erupting through the exit surface can result in an exit surface exhibiting far greater damage than the surface adjacent the entrance hole, making the use of pulsed or continuous wave high power lasers less attractive for drilling and/or cutting in situations where both precision entrance and exit holes are desired. Further, high volumetric material removal rates are typically achieved through the use of high laser pulse rates, which lead to considerable thermal and mechanical collateral damage, as discussed above. In addition, increasing power leads to plasma decoupling of the beam, e.g., incident laser energy is wasted in heating the ambient in front of the target. This is inherent to the process regardless of the laser type or wavelength chosen and thus leads to a manipulation of the energy within the applied laser pulse to yield higher material removal rates.
Additional possibilities for the application of lasers to the field of machining include the use of excimer lasers that emit high intensity pulses of ultraviolet (UV) light as cutting and/or drilling tools. Both the short wavelength characteristic of the UV light and the short nanosecond range pulse durations arising from the excimer lasers contribute to a different regime of laser-workpiece interaction. Short wavelength ultraviolet photons are energetic enough to directly break chemical bonds in a wider range of workpiece materials. As a consequence, UV excimer lasers can often vaporize a material target with minimal thermal energy transfer to adjacent workpiece material. The resultant ablatant (the vaporization product) is ejected away from the target surface, leaving the target relatively free from melt, recast, or other evidence of thermal damage. However, when used to drill precision holes, such high-energy laser pulses result in the exit hole being produced not by the laser pulses themselves but by molten and/or vaporized workpiece material exploding through the exit surface. The geyser of workpiece material erupting through the exit surface typically results in an exit surface exhibiting far greater damage than the entrance hole, detracting from the use of pulsed excimer lasers for drilling and/or cutting in situations where both precision entrance and exit holes are required. Further, high volumetric material removal rates are typically achieved through the use of high laser pulse rates, which lead to considerable thermal and mechanical collateral damage, as discussed above. However, it should be recognized that lasers in the UV wavelength machine some materials preferably to others, such as the polymers PFTE and PMMA versus the various steels, and the methods of UV laser machining are typically masks imaged onto the workpiece.
Laser machining tools have been used to machine organic, inorganic, metals and nonmetals such as ceramic materials, but have been largely commercially unsuccessful over the broad materials range due to their inability to produce the desired fine tolerances in commercial products such as valve seats, dies and molds and their tendency to degrade the substrate material due to the formation of microcracks. Typically, strength of the laser-machined parts is reduced considerably due to the formation of microcracks in the workpiece during the laser machining process. These microcracks are caused by thermal expansion and rapid cooling at the surface of the material exposed to and heated by the laser beam. These microcracks also serve as fracture initiators and result in fracturing or catastrophic failure of the workpiece during subsequent use.
Various other laser-machining techniques are known in the art. For instance, U.S. Pat. No. 4,638,145, issued Jan. 29, 1987, describes a laser machining apparatus for performing high quality cuts on plate type work pieces wherein the laser output is varied according to the traversing speed of the laser beam. The object is to minimize burn-through loss when machining soft steel workpieces. The output and velocity of the laser are controlled according to a predetermined formula dependent on the thickness and type of material. This referenced patent does not address the problem of precision machining of hard materials or permit the production of fine-machined finishes.
Currently, the lasers used for the bulk of machining or material processing applications are typically high-power solid-state lasers. These high-power solid-state lasers are typically used in a pulsed mode of operation for workpiece machining applications, such as cutting, welding and drilling. Ideally, lasers used for this purpose should have variable pulse lengths and variable pulse formats. For these applications, the pulse length typically selected is in the range of about 0.4 ms to about 1 ms, achieved through the duration of the applied pump source to the gain medium.
Typically, well-designed solid-state lasers produce pulses at a natural relaxation oscillation frequency when subjected to a short burst of pump energy. Various configurations have been proposed to provide control of the width, peak intensity, and spacing of laser relaxation output pulses. Control can be effected either by modulating the laser itself or by controlling laser pumping, which inputs energy to the laser cavity. Intracavity laser modulation usually requires the selective insertion of losses in the cavity to suppress lasing. A conventional Q-switch, for example, operates periodically to suppress lasing completely while the device continues to be pumped, and then suddenly removes the inserted loss and switches the laser on, which allows a large pulse to be emitted by the laser. When lasers are operated in a pulsed mode by means of conventional electro-optic (E.O.) Q-switching, the pulse length obtained is approximately 5-50 ns (nanoseconds), which is usually too short for most machining operations, and the pulses typically have a peak intensity that is too high for precision machining use. However, acousto-optic (A.O.) Q-switching results in longer length pulses that still exhibit high peak intensity, but are suitable for precision machining applications.
Conversely, free running, long pulse length lasers produce pulses with insufficient intensity for efficient precision cutting and drilling applications. Control of laser output by controlling the duration and timing of pumping energy also affords a degree of control of the output pulse waveform. For well-designed solid-state lasers, which produce output pulses at the natural relaxation oscillation (R.O.) frequency, control of the pumping duty cycle results in xe2x80x9cmacro-pulsesxe2x80x9d of laser output, each of which contains subpulses of rapidly decreasing intensity at the natural relaxation oscillation frequency. Solid-state lasers have typically been pumped by flash lamps, and now diodes, which, when pulsed, provide pumping energy that produces a laser pulse that varies widely in intensity over its time duration. Initially, the laser pulse is at peak intensity and then drops off toward the end of the pulse. These R.O. subpulses contained within this laser xe2x80x9cmacro-pulsexe2x80x9d have a variation in peak intensity and are, therefore, generally unsuitable for precision tool applications.
Various prior art patents, such as U.S. Pat Nos. 3,747,019 and 4,959,838, have disclosed relatively complex techniques for modulating the laser output to achieve a more desirably uniform sequence of output pulses. These techniques require some form of control system wherein the output beam is monitored and used to feed back a modulator control signal. Basically, the feedback control systems are needed because variations in the laser pump rate require commensurate variations in the modulation rate to maintain stable operation and produce the desired output pulse characteristics. However, sequential modulation of output pulses does not significantly improve the machining precision of the laser, especially with regard to the entrance surface topography of laser-drilled holes or laser cut sections (i.e., incident surface damage). For example, holes drilled or cut by conventional high-power solid-state lasers still suffer from xe2x80x9cdirtyxe2x80x9d entrance and exit surfaces formed as a result of molten or gaseous workpiece material redepositing upon the workpiece surface to form uneven, imprecise surface topography assocated with such a surface penetration mechanism. Likewise, laser welds formed through the use of conventional high-power solid state lasers also suffer from incident surface damage arising from the redepostoion of desolidified workpiece material around the weld seam, defining an upper limit to the precision to which the weld may be made. It will therefore be appreciated that there is still a significant need for improvement in the field of high-power lasers suitable for industrial use in precision drilling and machining applications. The present invention addresses this need.
The present invention relates to a method and apparatus for producing a machined workpiece. The method includes the steps of producing a laser pulse and directing the laser pulse throught the workpiece. The laser pulse is characterized by a first relatively low energy portion and at least two relatively high-energy spikes subsequent to the first relatively low energy portion.